PSI - Issue 6

Nikita Kazarinov et al. / Procedia Structural Integrity 6 (2017) 83–89 Nikita Kazarinov / Structural Integrity Procedia 00 (2017) 000–000

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Fig. 3. Caustic history with marked elastic waves and approximate crack tip path (a – 3.5 mm plate, b – 20 mm plate)

3. Results

Typical caustic history, obtained from the streak camera is shown in figure 3. The figure 3 depicts approximate dependence of the crack tip position on time, which roughly coincides with the caustic center. It is possible to compare velocity of the crack tip with velocities of longitudinal and transverse waves, measuring the inclines of the corresponding patterns. Dynamic loading resulted in almost monotonous growth of the crack for the 3.5 mm samples, while in case of thick 20 mm plates stepwise crack propagation was observed. The stepped trajectory shape was due to reflected waves, arriving from the sample edges. In general, crack tip covered much longer distance for the case of 20 mm plates. While for 3.5 mm samples crack extension ranged from 3.5 to 5 mm, the crack tip travelled up to 47 mm distance for the 20 mm samples. Differentiation of crack position by time provided crack tip velocity results. The corresponding dependencies are shown in figure 4. The observed oscillations should be treated as instantaneous crack velocities and averaged values should be assessed. Crack velocity values do not exceed the theoretical limit − 8 − Rayleigh wave velocity (Freund 1998). As seen from the figure 4, the crack velocity reaches its maximum, when crack starts to propagate, decreasing to zero. However, crack acceleration is observed for the case of the thick samples. Fluctuations of the crack velocity correspond to well-known patterns of the ruptured areas (Ravi-Chandar and Knauss 1984b) - fragmentary (large pieces), scaly, parabolic and mirror, which were found due to post-mortem investigation of the samples.